Open Data supplied by Natural Environment Research Council (NERC)

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

RRS Discovery 230 CTD Data Documentation

Introduction

Instrumentation and Methodology

Instrumentation Summary

The CTD profiles were taken with Neil Brown Systems MkIIIb/c CTDs (Deep01 and Deep02) mounted beneath a bottle rosette. Both CTDs are MkIIIb instruments converted to a MkIIIc format. Deep02 was specially modified to accept data from two FSI modules: one Platinum Resistance Thermometer Module FSI OTM-D-112 s/n 1325-011592, and one Conductivity Module FSI OCM-D-112 s/n 1333-011592. These mount on a specially modified 10 litre GO water bottle. The CTD was fitted with a dissolved oxygen sensor, a Chelsea Instruments fluorometer s/n 88/2050/95, a Chelsea Instruments Transmissometer s/n 161/2642/003, and a Simrad altimeter (model 807-200m).

Data Acquisition

Lowering rates for the CTD package were generally in the range 0.5-1.0ms -1 but could be up to 1.5ms -1 . CTD data were logged at 16 frames per second. The CTD deck unit passes raw data to a dedicated Level A microcomputer where 1 second averages are assembled. During this process the Level A calculates the rate of change of temperature and a median sorting routine detects and removes pressure spikes. These data are sent to the Level B for archival. The data are then passed to a Level C workstation for conversion to Pstar format and calibration.

A total of 143 stations were occupied. The first 135 stations were occupied using Deep01. For the remaining stations, 136 - 143, Deep02 was used. No CTD oxygen data were measured using Deep02.

Data Processing

The 1 second data passed to the Level C were converted to Pstar format and initially calibrated with coefficients from laboratory calibrations followed by a number of calibration corrections. The up and down cast data were extracted for merging with the bottle firing codes, thus the CTD variables were reconciled with the bottle samples. Final calibrations were applied using the sample bottle data. Finally, down cast data were extracted, sorted on pressure and averaged to 2db values.

The data were worked up to WOCE standards by the data originators before being supplied to BODC.

BODC Data Processing

No further calibrations were applied to the data received by BODC. BODC were mainly concerned with the screening and banking of the data.

The CTD data were received as 2db, pressure sorted, down cast data. Parameters were pressure (dbar), temperature (its-90), salinity (pss-78) and oxygen (µmol/kg). BODC have not received the fluorometer or transmissometer data.

The data were converted into the BODC internal format (a subset of NetCDF) to allow the use of in-house software tools, notably the graphics editor. Oxygen was converted to umol/l. Spikes in the data were manually flagged 'suspect' by modification of the associated quality control flag. In this way none of the original data values were edited or deleted during quality control.

The temperature, salinity and oxygen data from cruise D230 required little flagging and just a few points were set suspect. Profiles 093 and 102 were cold with low salinity. The oxygen profile for 001 has an unusual feature at ~2000db that has been flagged suspect.

Once screened, the CTD data were loaded into a database under the Oracle relational database management system. The start time stored in the database is the CTD deployment time, and the end time is the time the CTD was removed from the water. Actually these times are more precisely the start and end of data logging. Latitude and longitude are the mean positions between the start and end times calculated from the master navigation in the binary merged file.

References

World Ocean Circulation Experiment (WOCE)

The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

WOCE had two major goals:

Goal 1. To develop models to predict climate and to collect the data necessary to test them.

Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.

UK WOCE

The UK made a substantial contribution to the international World Ocean Circulation Experiment (WOCE) project by focusing on two important regions:

VIVALDI, a seven year programme of seasonally repeated surveys to study the upper ocean.

Long-term observations of ocean climate in the North West Approaches.

Satellite ocean surface topography, temperature and wind data were merged with in situ observations and models to create a complete description of ocean circulation, eddy motion and the way the ocean is driven by the atmosphere.

The surveys were forerunners to the international Global Ocean Observing System (GOOS). GOOS was later established to monitor annual to decadal changes in ocean circulation and heat storage which are vital in the prediction of climate change.

Fixed Station Information

Station Name

Extended Ellett Line

Category

Offshore route/traverse

Extended Ellett Line

The Extended Ellett Line is a hydrographic transect consisting of 64 individual fixed stations which have been occupied, typically on an annual basis, since September 1996. The Line runs from the south of Iceland, across the Iceland Basin to the outcrop of Rockall, and across the Rockall Trough to the north west coast of Scotland (see map). CTD dips and associated water sampling for the analysis of nutrients are routinely performed during each station occupation.

The Extended Ellett Line augments the original Ellett Line time series - a shorter repeated transect which encompassed those stations between Rockall and Scotland. Work on the Ellett Line was typically carried out at least once a year between 1975 and 1996.

Map of standard stations (1996-present)

Map produced using the GEBCO Digital Atlas

The white triangles indicate the nominal positions of the Extended Ellett Line stations visited since September 1996. Measurements made along the Extended Ellett Line lie within a box bounded by co-ordinates 56° N, 21° W at the south west corner and 65° N, 6° W at the north east corner.

Nominal Extended Ellett Line stations (September 1996-present)

Listed below are nominal details of the standard hydrographic stations that form the Extended Ellett Line. The majority of these stations have been sampled since the outset, although several have been added more recently.

Fixed Station Information

Station Name

Extended Ellett Line Station IB13

Category

Offshore location

Latitude

60° 30.00' N

Longitude

20° 0.00' W

Water depth below MSL

2500.0 m

Extended Ellett Line: Fixed Station IB13

Station IB13 is one of the fixed CTD stations, which together form The Extended Ellett Line. The line lies between Iceland and the Sound of Mull (Scotland) crossing the Iceland Basin and Rockall Trough via the outcrop of Rockall. As part of this initiative, CTD dips, together with associated discrete sampling of the water column, have typically been carried out annually at this station since September 1996.